1. Field of the Invention
The present invention relates generally to apparatus and method for power amplifying radio frequency (rf) or microwave rf signals. More particularly, the present invention pertains to an rf power amplifier in which two or more field-effect devices with selectively chosen DC bias circuits and rf decoupling circuits dividingly share a supply voltage, and a single rf output, two or more rf outputs, or two or more variably phase shifted rf outputs are produced.
2. Description of the Related Art
Gallium arsenide field-effect transistors (GaAsFETs) are the primary solid state devices used for amplification of high frequency signals in the range of 3 Ghz and higher. GaAsFETs have the advantages of being readily available and relatively inexpensive. However, a major disadvantage of GaAsFETs is that the maximum operating voltage is commonly +10.0 volts dc.
For many transmitter/amplifier applications, particularly airborne applications, the dc supply voltage is 28 volts dc, plus or minus 4.0 volts dc. Since gallium arsenide FETs have an operative voltage of +10 volts dc, the use of gallium arsenide FETs has presented a problem.
Traditionally, there have been two solutions to this problem. One is to use a linear voltage regulator. The other is to use a switching regulator.
In linear voltage regulators, the voltage is linearly regulated from the supply of 28 volts to approximately 10 volts with the power difference being dissipated in heat by the regulator. This type of regulation has the disadvantages of excessive heat and low power efficiency.
Switching regulators, on the other hand, are power converters that transfer the power of a higher voltage supply to lower voltage with increased current capacity. This type of regulation has the advantage of low heat dissipation and high power efficiency, but has the disadvantages of increased costs, space inefficiency due to large size, and the creation of a spurious signal on the rf carrier (EMI problems) due to the switching action of the regulator. A high-attenuation filter is required to suppress this spurious switching signal.
A third approach to solving the problem of disparity between the operating voltage of solid-state devices and a source voltage has been to connect the solid-state devices in series, thereby dividingly sharing the source voltage and utilizing the same current flow two or more times. This third approach was presented in IEEE Transactions on Microwave Theory and Techniques, Volume 46, Number 12, of December 1998, in an article entitled, “A 44-Ghz High IP3 InP-HBT Amplifier with Practical Current Reuse Biasing.”
This type of circuit solves the problem of the disparity between the operating voltage of solid-state devices and a higher supply voltage by stacking the solid-state devices in a totem pole fashion so that the source voltage is divided between the solid-state devices. Two, or more, solid-state devices are used in series for dc operation, but they are used in parallel for rf operation.
Thus, current that flows in series through the solid-state devices is used twice, or more times, in the production of the rf output. It is used once in each of two, or more, series-connected solid-state devices, thereby increasing the rf output for a given current flow, as compared to rf amplifiers connected in the conventional fashion.
However, totem-pole, voltage-dividing, or current-sharing circuits, have been used only at low rf powers, as in the above-referenced article wherein the power was in the order of 10 milliwatts. At higher rf powers, problems associated with inadequate rf decoupling have included low power efficiency, oscillation, a decrease in reliability of the circuits, and destruction of the solid-state devices.
In contrast, to the extremely low rf outputs in which the prior art has been able to utilize totem-pole circuitry, the present invention has been used with great success for rf outputs up to five Watts per solid-state device. However, this is not the limit, it is believed that the principles of the present invention may be used to make totem-pole circuits practical with solid-state devices with no apparent power limit.
In totem-pole circuits, problems with rf decoupling are most severe between the solid-state devices. In the present invention, the solid-state devices preferably are FETs. That is, when using FETs, rf decoupling is the most critical with regard to a source terminal of any FET that is connected to a drain terminal of a next-lower FET. Capacitors and rf chokes are used for rf decoupling and rf isolating, but selection and design of capacitor decoupling is the most critical.
The next most critical location for rf decoupling is the source terminal of the lower FET when the source terminal of the lower FET is connected to an electrical ground through a resistor, as shown herein. However, if a negative bias voltage is used for the gate of the lower FET, and the source is connected directly to an electrical ground, this source terminal is already rf decoupled. Again, capacitors are used for rf decoupling, and selection and design of capacitor decoupling is critical.
Other critical rf decoupling problems are those associated with the supply voltage to the drain of the upper FET and bias voltages to the gates of the FETs. The use of properly designed rf chokes are sufficient to provide adequate rf decoupling in these locations.
Unless rf decoupling is provided as taught herein, reduced efficiency will certainly occur, and both instability and destruction of the solid-state current devices are likely. More particularly, if one of the solid-state current devices goes into unstable self-oscillation, it will consume more dc bias and most likely become over biased resulting in destruction of the solid-state device.
In a totem-pole configuration that uses FETs, all FETs may be destroyed if one FET fails, depending on how the first FET fails. For example, if the upper FET oscillates and consumes the dc bias, it will be over biased and will be destroyed. If, in the destruction, the drain and source short circuit, which is a common type of failure, the lower FET will be over biased, too, so that the lower FET will fail also.
In short, inadequate rf decoupling, at the very least results in very low efficiency. At the worst, and with higher likelihood at higher rf outputs, it results in destruction of the FETs and/or damage or destruction of circuits connected to the FET inputs and outputs.